Lack of Simultaneity from Special Relativity.

In this example we have two people, one in a car travelling at a fast speed and an observer on the ground.

Here the car is crossing right in front of the observer.

Just as the car is right in front of the observer two bolts of lighting come down and strike near the front and the back of the car, equal distances tfrom where the observer and the person in the car line up.

The speed of light is constant, so the light from the bolts reach the observer at the same time because they travel the same distance. For the observer this event happens at the exact same time. However, for the person in the car the lighting bolt in front appears before the lightning bolt behind the car. The direction of the car shortens the distance the lightning bolt in front needs to travel. For the person in the car the lighting bolt in front existed first.

This shows that time is not a fixed reference frame. Events that appear to be simultaneous to one observer can occur at different times for another observer.